Japanese Unexamined Patent Application Publication No. H11-167132 discloses an optical system that generates ultraviolet light using a nonlinear optical crystal. Japanese Unexamined Patent Application Publication No. H11-167132 discloses that the atmosphere of the nonlinear optical crystal is 99.9% or more of nitrogen. Alternatively, in Japanese Unexamined Patent Application Publication No. H11-167132, the atmosphere of the nonlinear optical crystal is 99.9% of dry air.
Published Japanese Translation of PCT International Publication for Patent Application, No. 2015-536487 discloses a laser based illumination system that supplies humidified purge gas to a resonant cavity including a nonlinear crystal. Specifically, the purge gas flows through a humidity injection system to thereby be provided with water vapor. The purge gas is, for example, dry air free of contamination, nitrogen, or other combinations of inert gases.